1. Technical Field
The present invention relates to the field of electronics. More specifically, this invention relates to a device that allows the transfer of power from a grounded external source to an electrical apparatus in those instances where there is a limit on the ground current due to shock hazard.
In this disclosure, the term xe2x80x9cground currentxe2x80x9d refers to any current flowing through the ground connection of the external grounded source. This same current is also referred to as xe2x80x9ccommon mode current.xe2x80x9d The term xe2x80x9ccommon mode voltagexe2x80x9d refers to any voltage which can act as a source of ground current.
2. Background Information
Many electronic components today contain features for protection against electrical shock. Generally speaking, these features provide monitoring to determine if there is electricity discharging where it should not be, or leaking. When electrical leakage is detected, the features are designed to turn components off in order to protect against shock. In an instance such as charging of a battery, turning off of the components may prevent such desired charging. Accordingly, a need is recognized for a device that detects electrical leakage and is able to capture the leakage without turning other components off.
Referring to FIG. 1, an exemplary circuit for reducing ground current from an active load back through a main source is illustrated. Since FIG. 1 only illustrates an equivalent circuit for ground current, there may be additional sets of elements not shown therein. As shown, a grounded voltage source 12 is connected to a device 10, which in turn is connected to a load 18. The load 18 is considered to contain, in part, active circuitry such as switching devices that generate high frequency voltages with respect to the grounded enclosure, which in turn source current to the grounded enclosure through capacitance existing between the active circuitry and the grounded enclosure. FIG. 1 is solely intended to be the equivalent circuit for ground current and does not represent intended power flow between the source 12 and the load 18. For example, the exemplary circuit can be a charging system with the grounded voltage source 12 being an outlet, the device 10 being a filter for reducing load sourced ground current and the load 18 being a battery charger for charging a battery. The grounded voltage source 12 generates a common mode voltage that typically consists of low frequencies in value ranging from dc up to several hundred hertz (xe2x80x9cHz.xe2x80x9d). For example, if the source 12 is from a standard utility, the frequency of the common mode voltage would be 50 Hz in Europe and 60 Hz in the United States, if the source 12 is an airborne source the frequency could be 400 Hz. Similarly, a ground current is associated with the load with ground current consisting of higher frequency components typically ranging in value from several kHz to several MHz.
The load 18 must be powered from a grounded supply in such a way that the total ground current is kept below a given level to avoid a shock hazard. Moreover, circuits such as that shown in FIG. 1 typically contain a ground fault detector (not shown) that shuts down the device 10 if ground current is excessive. For example, if no filter was present in FIG. 1, the ground current from the source would be low and the ground current from the load would be high, causing the device 10 to shut down. Thus, as known in the art, a filter positioned between the voltage source 12 and the load 18 assists in keeping the ground current from the load 18 within the device 10. As illustrated, the filter consists of an inductor L1 14 and a capacitor C1 16. By increasing the capacitance of capacitor C1 16, the amount of ground current passing through the device 10 is reduced, or more specifically, the amount of ground current from the load 18 is reduced. However, by increasing the value of capacitor C1 16, the ground current from the common mode voltage source 12 into the device 10 increases, which can result in an unwanted discharge, e.g., a shock. The shock hazard results from the return line (not shown) containing a voltage higher than ground.
Thus, there is an optimum capacitance value for capacitor C1, resulting in a minimum value of ground current from the common mode voltage source 12. However, in some situations this minimum value of ground current from the common mode voltage source 12 is still above a maximum allowable level. Accordingly, there is needed a device which keeps the load ground current enclosed within the apparatus without limiting the total amount of filtering that may be achieved.
In the disclosed embodiment, the present invention alleviates the drawbacks described above with respect to the transfer of power without exceeding a safe level of ground current. The present invention utilizes a controlled voltage source in series with a capacitor of a filter for canceling or substantially reducing ground current from an external power source and reducing the ground current from a load.
In one embodiment, a device is inserted between a grounded poly-phase external voltage source and a load providing low insertion loss for differential mode currents and high insertion loss for ground current. The device includes at least one voltage line, a filter, a summing block, and a ground referenced controlled voltage source. The at least one voltage line receives at least one voltage from the grounded poly-phase external voltage source with the at least one voltage having a common mode voltage. The filter is connected to each of the at least one voltage lines and is configured to reduce the ground current from the load. The summing block is configured to add the voltages from each of the at least one voltage lines to generate a total common mode voltage. The ground referenced controlled voltage source generates a cancellation voltage equal to the total common mode voltage in response to the generation of the total common mode voltage. The total common mode voltage is injected into the filter and substantially reduces the common mode voltage on each voltage line thereby substantially reducing ground currents associated with each common mode voltage on each voltage line.
In one embodiment, a method for providing low insertion loss for differential mode currents and high insertion loss for ground currents for a device between a grounded poly-phase external voltage source and a load. The method includes the steps of receiving at least one voltage from the external source via at least one voltage line, reducing the ground current from the load using a filter, summing the voltage from the at least one voltage line, generating a cancellation common mode voltage equal to the sum of the summed voltages, and injecting the cancellation common mode voltage into the filter. By injecting the cancellation common mode voltage into the filter, the common mode voltage on each voltage line is substantially reduced which substantially reduces the ground current on each voltage line.
The general beneficial effects described above apply generally to each of the exemplary descriptions and characterizations of the devices and mechanisms disclosed herein. The specific structures through which these benefits are delivered will be described in detail herein below.